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          Reflect
        </h1> <div class="page-slot page-slot-top"><!-- 固定100% * 90px可显示，max-height:90px未见显示-->
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          (adsbygoogle = window.adsbygoogle || []).push({});
      </script></div> <div class="theme-vdoing-content content__default"><h1 id="reflect"><a href="#reflect" class="header-anchor">#</a> Reflect</h1> <h2 id="概述"><a href="#概述" class="header-anchor">#</a> 概述</h2> <p><code>Reflect</code>对象与<code>Proxy</code>对象一样，也是 ES6 为了操作对象而提供的新 API。<code>Reflect</code>对象的设计目的有这样几个。</p> <p>（1） **将<code>Object</code>对象的一些明显属于语言内部的方法（比如<code>Object.defineProperty</code>），放到<code>Reflect</code>对象上。**现阶段，某些方法同时在<code>Object</code>和<code>Reflect</code>对象上部署，未来的新方法将只部署在<code>Reflect</code>对象上。也就是说，从<code>Reflect</code>对象上可以拿到语言内部的方法。

（2） <strong>修改某些<code>Object</code>方法的返回结果，让其变得更合理</strong>。比如，<code>Object.defineProperty(obj, name, desc)</code>在无法定义属性时，会抛出一个错误，而<code>Reflect.defineProperty(obj, name, desc)</code>则会返回<code>false</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 老写法</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
  Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> property<span class="token punctuation">,</span> attributes<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// success</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// failure</span>
<span class="token punctuation">}</span>

<span class="token comment">// 新写法</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>Reflect<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> property<span class="token punctuation">,</span> attributes<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// success</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
  <span class="token comment">// failure</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>（3） <strong>让<code>Object</code>操作都变成函数行为</strong>。某些<code>Object</code>操作是命令式，比如<code>name in obj</code>和<code>delete obj[name]</code>，而<code>Reflect.has(obj, name)</code>和<code>Reflect.deleteProperty(obj, name)</code>让它们变成了函数行为。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 老写法</span>
<span class="token string">'assign'</span> <span class="token keyword">in</span> Object <span class="token comment">// true</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">has</span><span class="token punctuation">(</span>Object<span class="token punctuation">,</span> <span class="token string">'assign'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>（4）<code>Reflect</code>对象的方法与<code>Proxy</code>对象的方法一一对应，只要是<code>Proxy</code>对象的方法，就能在<code>Reflect</code>对象上找到对应的方法。这就让<code>Proxy</code>对象可以方便地调用对应的<code>Reflect</code>方法，完成默认行为，作为修改行为的基础。也就是说，<strong>不管<code>Proxy</code>怎么修改默认行为，你总可以在<code>Reflect</code>上获取默认行为</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">Proxy</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token function-variable function">set</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> name<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">var</span> success <span class="token operator">=</span> Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> name<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>success<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'property '</span> <span class="token operator">+</span> name <span class="token operator">+</span> <span class="token string">' on '</span> <span class="token operator">+</span> target <span class="token operator">+</span> <span class="token string">' set to '</span> <span class="token operator">+</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> success<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>Proxy</code>方法拦截<code>target</code>对象的属性赋值行为。它采用<code>Reflect.set</code>方法将值赋值给对象的属性，确保完成原有的行为，然后再部署额外的功能。</p> <p>下面是另一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> loggedObj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> name<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'get'</span><span class="token punctuation">,</span> target<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token function">deleteProperty</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> name</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'delete'</span> <span class="token operator">+</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> Reflect<span class="token punctuation">.</span><span class="token function">deleteProperty</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token function">has</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> name</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'has'</span> <span class="token operator">+</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> Reflect<span class="token punctuation">.</span><span class="token function">has</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>上面代码中，每一个<code>Proxy</code>对象的拦截操作（<code>get</code>、<code>delete</code>、<code>has</code>），内部都调用对应的<code>Reflect</code>方法，保证原生行为能够正常执行。添加的工作，就是将每一个操作输出一行日志。</p> <p>有了<code>Reflect</code>对象以后，很多操作会更易读。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 老写法</span>
<span class="token class-name">Function</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span>floor<span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">1.75</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>

<span class="token comment">// 新写法</span>
<span class="token function">Reflect</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span>floor<span class="token punctuation">,</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token number">1.75</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h2 id="静态方法"><a href="#静态方法" class="header-anchor">#</a> 静态方法</h2> <p><code>Reflect</code>对象一共有 13 个静态方法。</p> <ul><li>Reflect.apply(target, thisArg, args)</li> <li>Reflect.construct(target, args)</li> <li>Reflect.get(target, name, receiver)</li> <li>Reflect.set(target, name, value, receiver)</li> <li>Reflect.defineProperty(target, name, desc)</li> <li>Reflect.deleteProperty(target, name)</li> <li>Reflect.has(target, name)</li> <li>Reflect.ownKeys(target)</li> <li>Reflect.isExtensible(target)</li> <li>Reflect.preventExtensions(target)</li> <li>Reflect.getOwnPropertyDescriptor(target, name)</li> <li>Reflect.getPrototypeOf(target)</li> <li>Reflect.setPrototypeOf(target, prototype)</li></ul> <p>上面这些方法的作用，大部分与<code>Object</code>对象的同名方法的作用都是相同的，而且它与<code>Proxy</code>对象的方法是一一对应的。下面是对它们的解释。</p> <h3 id="reflect-get-target-name-receiver"><a href="#reflect-get-target-name-receiver" class="header-anchor">#</a> Reflect.get(target, name, receiver)</h3> <p><code>Reflect.get</code>方法查找并返回<code>target</code>对象的<code>name</code>属性，如果没有该属性，则返回<code>undefined</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span>
  foo<span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
  bar<span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span>
  <span class="token keyword">get</span> <span class="token function">baz</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>foo <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>bar<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span>

Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'bar'</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'baz'</span><span class="token punctuation">)</span> <span class="token comment">// 3</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>如果<code>name</code>属性部署了读取函数（getter），则读取函数的<code>this</code>绑定<code>receiver</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span>
  foo<span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
  bar<span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span>
  <span class="token keyword">get</span> <span class="token function">baz</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>foo <span class="token operator">+</span> <span class="token keyword">this</span><span class="token punctuation">.</span>bar<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">var</span> myReceiverObject <span class="token operator">=</span> <span class="token punctuation">{</span>
  foo<span class="token operator">:</span> <span class="token number">4</span><span class="token punctuation">,</span>
  bar<span class="token operator">:</span> <span class="token number">4</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'baz'</span><span class="token punctuation">,</span> myReceiverObject<span class="token punctuation">)</span> <span class="token comment">// 8</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>如果第一个参数不是对象，<code>Reflect.get</code>方法会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
Reflect<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="reflect-set-target-name-value-receiver"><a href="#reflect-set-target-name-value-receiver" class="header-anchor">#</a> Reflect.set(target, name, value, receiver)</h3> <p><code>Reflect.set</code>方法设置<code>target</code>对象的<code>name</code>属性等于<code>value</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span>
  foo<span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
  <span class="token keyword">set</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>foo <span class="token operator">=</span> value<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span>

myObject<span class="token punctuation">.</span>foo <span class="token comment">// 1</span>

Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
myObject<span class="token punctuation">.</span>foo <span class="token comment">// 2</span>

Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'bar'</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span>
myObject<span class="token punctuation">.</span>foo <span class="token comment">// 3</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>如果<code>name</code>属性设置了赋值函数，则赋值函数的<code>this</code>绑定<code>receiver</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span>
  foo<span class="token operator">:</span> <span class="token number">4</span><span class="token punctuation">,</span>
  <span class="token keyword">set</span> <span class="token function">bar</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>foo <span class="token operator">=</span> value<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">var</span> myReceiverObject <span class="token operator">=</span> <span class="token punctuation">{</span>
  foo<span class="token operator">:</span> <span class="token number">0</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'bar'</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> myReceiverObject<span class="token punctuation">)</span><span class="token punctuation">;</span>
myObject<span class="token punctuation">.</span>foo <span class="token comment">// 4</span>
myReceiverObject<span class="token punctuation">.</span>foo <span class="token comment">// 1</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>注意，如果 <code>Proxy</code>对象和 <code>Reflect</code>对象联合使用，前者拦截赋值操作，后者完成赋值的默认行为，而且传入了<code>receiver</code>，那么<code>Reflect.set</code>会触发<code>Proxy.defineProperty</code>拦截。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> p <span class="token operator">=</span> <span class="token punctuation">{</span>
  a<span class="token operator">:</span> <span class="token string">'a'</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token function">set</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'set'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> attribute</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'defineProperty'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    Reflect<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> attribute<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>p<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token string">'A'</span><span class="token punctuation">;</span>
<span class="token comment">// set</span>
<span class="token comment">// defineProperty</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><p>上面代码中，<code>Proxy.set</code>拦截里面使用了<code>Reflect.set</code>，而且传入了<code>receiver</code>，导致触发<code>Proxy.defineProperty</code>拦截。这是因为<code>Proxy.set</code>的<code>receiver</code>参数总是指向当前的 <code>Proxy</code>实例（即上例的<code>obj</code>），而<code>Reflect.set</code>一旦传入<code>receiver</code>，就会将属性赋值到<code>receiver</code>上面（即<code>obj</code>），导致触发<code>defineProperty</code>拦截。如果<code>Reflect.set</code>没有传入<code>receiver</code>，那么就不会触发<code>defineProperty</code>拦截。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> p <span class="token operator">=</span> <span class="token punctuation">{</span>
  a<span class="token operator">:</span> <span class="token string">'a'</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> handler <span class="token operator">=</span> <span class="token punctuation">{</span>
  <span class="token function">set</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'set'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">)</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> attribute</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'defineProperty'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    Reflect<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> attribute<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">let</span> obj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>p<span class="token punctuation">,</span> handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
obj<span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token string">'A'</span><span class="token punctuation">;</span>
<span class="token comment">// set</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><p>如果第一个参数不是对象，<code>Reflect.set</code>会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="reflect-has-obj-name"><a href="#reflect-has-obj-name" class="header-anchor">#</a> Reflect.has(obj, name)</h3> <p><code>Reflect.has</code>方法对应<code>name in obj</code>里面的<code>in</code>运算符。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span>
  foo<span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// 旧写法</span>
<span class="token string">'foo'</span> <span class="token keyword">in</span> myObject <span class="token comment">// true</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">has</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>如果<code>Reflect.has()</code>方法的第一个参数不是对象，会报错。</p> <h3 id="reflect-deleteproperty-obj-name"><a href="#reflect-deleteproperty-obj-name" class="header-anchor">#</a> Reflect.deleteProperty(obj, name)</h3> <p><code>Reflect.deleteProperty</code>方法等同于<code>delete obj[name]</code>，用于删除对象的属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> myObj <span class="token operator">=</span> <span class="token punctuation">{</span> foo<span class="token operator">:</span> <span class="token string">'bar'</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// 旧写法</span>
<span class="token keyword">delete</span> myObj<span class="token punctuation">.</span>foo<span class="token punctuation">;</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">deleteProperty</span><span class="token punctuation">(</span>myObj<span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>该方法返回一个布尔值。如果删除成功，或者被删除的属性不存在，返回<code>true</code>；删除失败，被删除的属性依然存在，返回<code>false</code>。</p> <p>如果<code>Reflect.deleteProperty()</code>方法的第一个参数不是对象，会报错。</p> <h3 id="reflect-construct-target-args"><a href="#reflect-construct-target-args" class="header-anchor">#</a> Reflect.construct(target, args)</h3> <p><code>Reflect.construct</code>方法等同于<code>new target(...args)</code>，这提供了一种不使用<code>new</code>，来调用构造函数的方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">Greeting</span><span class="token punctuation">(</span><span class="token parameter">name</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// new 的写法</span>
<span class="token keyword">const</span> instance <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Greeting</span><span class="token punctuation">(</span><span class="token string">'张三'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// Reflect.construct 的写法</span>
<span class="token keyword">const</span> instance <span class="token operator">=</span> Reflect<span class="token punctuation">.</span><span class="token function">construct</span><span class="token punctuation">(</span>Greeting<span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token string">'张三'</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>如果<code>Reflect.construct()</code>方法的第一个参数不是函数，会报错。</p> <h3 id="reflect-getprototypeof-obj"><a href="#reflect-getprototypeof-obj" class="header-anchor">#</a> Reflect.getPrototypeOf(obj)</h3> <p><code>Reflect.getPrototypeOf</code>方法用于读取对象的<code>__proto__</code>属性，对应<code>Object.getPrototypeOf(obj)</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> myObj <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FancyThing</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 旧写法</span>
Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>myObj<span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token class-name">FancyThing</span><span class="token punctuation">.</span>prototype<span class="token punctuation">;</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span>myObj<span class="token punctuation">)</span> <span class="token operator">===</span> <span class="token class-name">FancyThing</span><span class="token punctuation">.</span>prototype<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p><code>Reflect.getPrototypeOf</code>和<code>Object.getPrototypeOf</code>的一个区别是，如果参数不是对象，<code>Object.getPrototypeOf</code>会将这个参数转为对象，然后再运行，而<code>Reflect.getPrototypeOf</code>会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// Number {[[PrimitiveValue]]: 0}</span>
Reflect<span class="token punctuation">.</span><span class="token function">getPrototypeOf</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="reflect-setprototypeof-obj-newproto"><a href="#reflect-setprototypeof-obj-newproto" class="header-anchor">#</a> Reflect.setPrototypeOf(obj, newProto)</h3> <p><code>Reflect.setPrototypeOf</code>方法用于设置目标对象的原型（prototype），对应<code>Object.setPrototypeOf(obj, newProto)</code>方法。它返回一个布尔值，表示是否设置成功。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> myObj <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// 旧写法</span>
Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>myObj<span class="token punctuation">,</span> <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>myObj<span class="token punctuation">,</span> <span class="token class-name">Array</span><span class="token punctuation">.</span>prototype<span class="token punctuation">)</span><span class="token punctuation">;</span>

myObj<span class="token punctuation">.</span>length <span class="token comment">// 0</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>如果无法设置目标对象的原型（比如，目标对象禁止扩展），<code>Reflect.setPrototypeOf</code>方法返回<code>false</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Reflect<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token comment">// true</span>
Reflect<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span>Object<span class="token punctuation">.</span><span class="token function">freeze</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token comment">// false</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>如果第一个参数不是对象，<code>Object.setPrototypeOf</code>会返回第一个参数本身，而<code>Reflect.setPrototypeOf</code>会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// 1</span>

Reflect<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// TypeError: Reflect.setPrototypeOf called on non-object</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>如果第一个参数是<code>undefined</code>或<code>null</code>，<code>Object.setPrototypeOf</code>和<code>Reflect.setPrototypeOf</code>都会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// TypeError: Object.setPrototypeOf called on null or undefined</span>

Reflect<span class="token punctuation">.</span><span class="token function">setPrototypeOf</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token comment">// TypeError: Reflect.setPrototypeOf called on non-object</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h3 id="reflect-apply-func-thisarg-args"><a href="#reflect-apply-func-thisarg-args" class="header-anchor">#</a> Reflect.apply(func, thisArg, args)</h3> <p><code>Reflect.apply</code>方法等同于<code>Function.prototype.apply.call(func, thisArg, args)</code>，用于绑定<code>this</code>对象后执行给定函数。</p> <p>一般来说，如果要绑定一个函数的<code>this</code>对象，可以这样写<code>fn.apply(obj, args)</code>，但是如果函数定义了自己的<code>apply</code>方法，就只能写成<code>Function.prototype.apply.call(fn, obj, args)</code>，采用<code>Reflect</code>对象可以简化这种操作。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> ages <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">11</span><span class="token punctuation">,</span> <span class="token number">33</span><span class="token punctuation">,</span> <span class="token number">12</span><span class="token punctuation">,</span> <span class="token number">54</span><span class="token punctuation">,</span> <span class="token number">18</span><span class="token punctuation">,</span> <span class="token number">96</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token comment">// 旧写法</span>
<span class="token keyword">const</span> youngest <span class="token operator">=</span> Math<span class="token punctuation">.</span><span class="token function">min</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>Math<span class="token punctuation">,</span> ages<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> oldest <span class="token operator">=</span> Math<span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>Math<span class="token punctuation">,</span> ages<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> type <span class="token operator">=</span> <span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>youngest<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 新写法</span>
<span class="token keyword">const</span> youngest <span class="token operator">=</span> <span class="token function">Reflect</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span>min<span class="token punctuation">,</span> Math<span class="token punctuation">,</span> ages<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> oldest <span class="token operator">=</span> <span class="token function">Reflect</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span>max<span class="token punctuation">,</span> Math<span class="token punctuation">,</span> ages<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> type <span class="token operator">=</span> <span class="token function">Reflect</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token class-name">Object</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span>toString<span class="token punctuation">,</span> youngest<span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><h3 id="reflect-defineproperty-target-propertykey-attributes"><a href="#reflect-defineproperty-target-propertykey-attributes" class="header-anchor">#</a> Reflect.defineProperty(target, propertyKey, attributes)</h3> <p><code>Reflect.defineProperty</code>方法基本等同于<code>Object.defineProperty</code>，用来为对象定义属性。未来，后者会被逐渐废除，请从现在开始就使用<code>Reflect.defineProperty</code>代替它。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">MyDate</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">/*…*/</span>
<span class="token punctuation">}</span>

<span class="token comment">// 旧写法</span>
Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>MyDate<span class="token punctuation">,</span> <span class="token string">'now'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token function-variable function">value</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> Date<span class="token punctuation">.</span><span class="token function">now</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>MyDate<span class="token punctuation">,</span> <span class="token string">'now'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token function-variable function">value</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> Date<span class="token punctuation">.</span><span class="token function">now</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>如果<code>Reflect.defineProperty</code>的第一个参数不是对象，就会抛出错误，比如<code>Reflect.defineProperty(1, 'foo')</code>。</p> <p>这个方法可以与<code>Proxy.defineProperty</code>配合使用。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token function">defineProperty</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> prop<span class="token punctuation">,</span> descriptor</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>descriptor<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> Reflect<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> prop<span class="token punctuation">,</span> descriptor<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

p<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
<span class="token comment">// {value: &quot;bar&quot;, writable: true, enumerable: true, configurable: true}</span>

p<span class="token punctuation">.</span>foo <span class="token comment">// &quot;bar&quot;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>上面代码中，<code>Proxy.defineProperty</code>对属性赋值设置了拦截，然后使用<code>Reflect.defineProperty</code>完成了赋值。</p> <h3 id="reflect-getownpropertydescriptor-target-propertykey"><a href="#reflect-getownpropertydescriptor-target-propertykey" class="header-anchor">#</a> Reflect.getOwnPropertyDescriptor(target, propertyKey)</h3> <p><code>Reflect.getOwnPropertyDescriptor</code>基本等同于<code>Object.getOwnPropertyDescriptor</code>，用于得到指定属性的描述对象，将来会替代掉后者。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'hidden'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
  value<span class="token operator">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
  enumerable<span class="token operator">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 旧写法</span>
<span class="token keyword">var</span> theDescriptor <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'hidden'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 新写法</span>
<span class="token keyword">var</span> theDescriptor <span class="token operator">=</span> Reflect<span class="token punctuation">.</span><span class="token function">getOwnPropertyDescriptor</span><span class="token punctuation">(</span>myObject<span class="token punctuation">,</span> <span class="token string">'hidden'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p><code>Reflect.getOwnPropertyDescriptor</code>和<code>Object.getOwnPropertyDescriptor</code>的一个区别是，如果第一个参数不是对象，<code>Object.getOwnPropertyDescriptor(1, 'foo')</code>不报错，返回<code>undefined</code>，而<code>Reflect.getOwnPropertyDescriptor(1, 'foo')</code>会抛出错误，表示参数非法。</p> <h3 id="reflect-isextensible-target"><a href="#reflect-isextensible-target" class="header-anchor">#</a> Reflect.isExtensible (target)</h3> <p><code>Reflect.isExtensible</code>方法对应<code>Object.isExtensible</code>，返回一个布尔值，表示当前对象是否可扩展。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// 旧写法</span>
Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>myObject<span class="token punctuation">)</span> <span class="token comment">// true</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span>myObject<span class="token punctuation">)</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>如果参数不是对象，<code>Object.isExtensible</code>会返回<code>false</code>，因为非对象本来就是不可扩展的，而<code>Reflect.isExtensible</code>会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Object<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
Reflect<span class="token punctuation">.</span><span class="token function">isExtensible</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="reflect-preventextensions-target"><a href="#reflect-preventextensions-target" class="header-anchor">#</a> Reflect.preventExtensions(target)</h3> <p><code>Reflect.preventExtensions</code>对应<code>Object.preventExtensions</code>方法，用于让一个对象变为不可扩展。它返回一个布尔值，表示是否操作成功。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// 旧写法</span>
Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>myObject<span class="token punctuation">)</span> <span class="token comment">// Object {}</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span>myObject<span class="token punctuation">)</span> <span class="token comment">// true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>如果参数不是对象，<code>Object.preventExtensions</code>在 ES5 环境报错，在 ES6 环境返回传入的参数，而<code>Reflect.preventExtensions</code>会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES5 环境</span>
Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>

<span class="token comment">// ES6 环境</span>
Object<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">preventExtensions</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><h3 id="reflect-ownkeys-target"><a href="#reflect-ownkeys-target" class="header-anchor">#</a> Reflect.ownKeys (target)</h3> <p><code>Reflect.ownKeys</code>方法用于返回对象的所有属性，基本等同于<code>Object.getOwnPropertyNames</code>与<code>Object.getOwnPropertySymbols</code>之和。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">var</span> myObject <span class="token operator">=</span> <span class="token punctuation">{</span>
  foo<span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">,</span>
  bar<span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span><span class="token function">for</span><span class="token punctuation">(</span><span class="token string">'baz'</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token operator">:</span> <span class="token number">3</span><span class="token punctuation">,</span>
  <span class="token punctuation">[</span>Symbol<span class="token punctuation">.</span><span class="token function">for</span><span class="token punctuation">(</span><span class="token string">'bing'</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token operator">:</span> <span class="token number">4</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// 旧写法</span>
Object<span class="token punctuation">.</span><span class="token function">getOwnPropertyNames</span><span class="token punctuation">(</span>myObject<span class="token punctuation">)</span>
<span class="token comment">// ['foo', 'bar']</span>

Object<span class="token punctuation">.</span><span class="token function">getOwnPropertySymbols</span><span class="token punctuation">(</span>myObject<span class="token punctuation">)</span>
<span class="token comment">//[Symbol(baz), Symbol(bing)]</span>

<span class="token comment">// 新写法</span>
Reflect<span class="token punctuation">.</span><span class="token function">ownKeys</span><span class="token punctuation">(</span>myObject<span class="token punctuation">)</span>
<span class="token comment">// ['foo', 'bar', Symbol(baz), Symbol(bing)]</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p>如果<code>Reflect.ownKeys()</code>方法的第一个参数不是对象，会报错。</p> <h2 id="实例-使用-proxy-实现观察者模式"><a href="#实例-使用-proxy-实现观察者模式" class="header-anchor">#</a> 实例：使用 Proxy 实现观察者模式</h2> <p><strong>观察者模式（Observer mode）指的是函数自动观察数据对象，一旦对象有变化，函数就会自动执行</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> person <span class="token operator">=</span> <span class="token function">observable</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
  name<span class="token operator">:</span> <span class="token string">'张三'</span><span class="token punctuation">,</span>
  age<span class="token operator">:</span> <span class="token number">20</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 观察目标</span>

<span class="token keyword">function</span> <span class="token function">print</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>person<span class="token punctuation">.</span>name<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">, </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>person<span class="token punctuation">.</span>age<span class="token interpolation-punctuation punctuation">}</span></span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span>
<span class="token punctuation">}</span> <span class="token comment">// 观察者</span>

<span class="token function">observe</span><span class="token punctuation">(</span>print<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 启动观察</span>
person<span class="token punctuation">.</span>name <span class="token operator">=</span> <span class="token string">'李四'</span><span class="token punctuation">;</span>
<span class="token comment">// 输出</span>
<span class="token comment">// 李四, 20</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码中，数据对象<code>person</code>是观察目标，函数<code>print</code>是观察者。一旦数据对象发生变化，<code>print</code>就会自动执行。</p> <p>下面，使用 Proxy 写一个观察者模式的最简单实现，即实现<code>observable</code>和<code>observe</code>这两个函数。思路是<code>observable</code>函数返回一个原始对象的 Proxy 代理，拦截赋值操作，触发充当观察者的各个函数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> queuedObservers <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Set</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> <span class="token function-variable function">observe</span> <span class="token operator">=</span> <span class="token parameter">fn</span> <span class="token operator">=&gt;</span> queuedObservers<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>fn<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token function-variable function">observable</span> <span class="token operator">=</span> <span class="token parameter">obj</span> <span class="token operator">=&gt;</span> <span class="token keyword">new</span> <span class="token class-name">Proxy</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> <span class="token punctuation">{</span>set<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">set</span><span class="token punctuation">(</span><span class="token parameter">target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> result <span class="token operator">=</span> Reflect<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>target<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> receiver<span class="token punctuation">)</span><span class="token punctuation">;</span>
  queuedObservers<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token parameter">observer</span> <span class="token operator">=&gt;</span> <span class="token function">observer</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> result<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面代码中，先定义了一个<code>Set</code>集合，所有观察者函数都放进这个集合。然后，<code>observable</code>函数返回原始对象的代理，拦截赋值操作。拦截函数<code>set</code>之中，会自动执行所有观察者。</p></div></div> <div class="page-slot page-slot-bottom"><!-- 横向自适应 -->
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